Overview and first year observation of CALET Gamma ray Burst Monitor
Yuta Kawakubo,1 Atsumasa Yoshida,1 Takanori Sakamoto,1 for the CALET collaboration
1Department of Physics and Mathematics, Aoyama Gakuin University
5-10-1 Fuchinobe, Chuo, Sagamihara, Kanagawa 252-5258, Japan
E-mail(YK):ykawakubo@phys.aoyama.ac.jp
Abstract

Calorimetric electron telescope (CALET) was launched by the 5th H-IIB/HTV on August 19, 2015.
CALET includes CALET Gamma ray Burst Monitor (CGBM) to observe GRB prompt emissions. CGBM
consists of two Hard X-ray Monitors (HXM) and Soft Gamma ray Monitor (SGM). CGBM is able toobserve prompt emissions of GRBs in the 7 keV - 20 MeV band (HXM; 7 keV - 1 MeV, SGM; 40 keV 20 MeV). CALET started its nominal observation on October 8, 2015. So far, CGBM detected 49 GRBs
which include 9 short GRBs and 40 long GRBs. The detection rate is about 4 GRBs per month. Four
GRBs are also detected by MAXI. The distribution of T90 is similar to GRBs detected by BATSE. The
mean of T90 measured by CGBM is 8.9 s.
Key words: GRB — CALET
1. Introduction
A gamma ray burst (GRB) is a mysterious explosionin the universe. A prompt emission, an explosive main
emission of GRBs, is very bright and variable. Although
the mechanism of a prompt emission is not well understood, a cogent scenario is the internal shock model.
That is collisions between each ejecta which have a different velocity in a relativistic jet cause the internalshock (Rees and Meszaros, 1994 ). The shock accelerated electrons radiate X-rays or gamma-rays due to thesynchrotron radiation. The spectra of the prompt emissions show a broad energy distribution from keV to MeV
range. These spectra are well represented by a smoothly
jointed broken power-law, namely Band function (Bandet al. 1993 ). To study its wide band nature, spectral
observations of prompt emissions need a broad energy
coverage. Fermi/LAT detected very high energy photons above 100 MeV from GRBs and found out that theycame later than typical hard X-ray photons of prompt
emission (Abdo et al. 2009 ). The high energy photons
are thought to be caused by an external shock whicharises when the ejecta from the GRB collides with interstellar medium.
2. CALorimetric Electron Telescope (CALET)
A main purpose of CALorimetric Electron Telescope(CALET) mission is observations of cosmic electronsand gamma-rays in the GeV - TeV energy band (Toriiet al. 2011 ). CALET also aims to observe high energy transients such as GRBs in X-ray and gamma-Fig. 1. The image of CALET. CALET is equipped with one SGM andtwo HXMs.

ray range. CALET carries two scientific instruments
CALorimetor (CAL) and CALET Gamma ray BurstMonitor (CGBM). CAL has a sensitivity for gamma-raysin the 1 GeV - 10 TeV band. CGBM utilizes two typeof scintillation detectors : Hard X-ray Monitor (HXM;
7 keV - 1 MeV) and Soft Gamma ray Monitor (SGM;
40 keV - 20 MeV). All sensors of CGBM have two amplifiers with diﬀerent gain (high / low) to observe GRB
radiations in the broad energy range.
Table 1. Specification of HXM and SGM
Number of detector
Detector
Diameter [mm]Thickness [mm]
Energy range [keV]
Field of view

HXM2LaBr3 (Ce)6112.77-1000
∼3 sr

SGM
1BGO
10276100-20000∼8 sr

Table 2. Data of CGBM

Data
Time History (TH)Pulse Height (PH)Event data (EVT)

Channels (High)41024096

Channels (Low)44104096

Time resolution
0.125s4s
62.5µsCALET was launched by the 5th H-IIB/HTV on August 19, 2015 and attached on the JEM module of theinternational space station (ISS) on August 25. The initial checkout was performed on September 22. The 72
hour running test was carried out from October 5 to 8.
During this test, CGBM detected GRB 151006A which
was the first GRB detection by CGBM. CGBM started
the nominal observation on October 8. Since CGBM
high voltages are turned oﬀ at the SAA region and highlatitude regions to protect PMT and to reduce false trigger alerts, the observation eﬃciency is about 60%.
4. Observation
CGBM detected 49 GRBs, 40 long bursts and 9 shortbursts, during October 2015 to November 2016. The
detection rate of GRBs is ∼4 GRBs per month. The
most of the CGBM GRBs were also detected byother instruments (e.g. Fermi/GBM, INTEGRAL/ACS,KONUS/Wind, Swift/BAT, MAXI/GSC). Especially, 4
GRBs were also detected by MAXI/GSC. The T90 durations of GRBs detected by CGBM were measured by
SGM in the 40 - 1000 keV band. We classified GRBs
based on the T90 smaller than 2 s as short bursts. Fig. 2
shows the T90 distributions of GRBs detected by CGBM.
The mean of T90 of the CGBM GRBs is 8.9 s a bit
smaller than that of 9.3 s by BATSE (see Fig. 2). Since
the T90 is measured at diﬀerent energy bands by diﬀerent instruments, the measured T90 could be aﬀected by
the energy dependence of GRB pulses (Fenimore et al.
1995 ) and the sensitivity of instruments.
CALET joins to the LIGO/Virgo electro-magnetic follow up program. When GW 150914 occurred, CGBM

64CGBM
40 – 1000 keV

2

Number of bursts

3. Data collection and In-orbit operation
CGBM regularly collects two kinds of monitor data. Oneis Pulse Height (PH) data. The other is Time History (TH) data. CGBM has the on-board trigger system which calculates a signal-to-noise ratio every 0.25
s. If the signal-to-noise ratio exceeds a trigger threshold,
CGBM captures event data with a time resolution of 62.5
µs. We setup a pipeline to send an alert to Gamma-ray
Coordinates Network automatically. When the CGBM
on-board trigger happens, CAL decreases the lower energy thresholds for gamma ray from 10 GeV to 1 GeV.
Advanced Stellar Compass (ASC) which is a optical starcamera takes two images.

0100

BATSE
25 – 350 keV

500200Fermi/GBM
50 – 300 keV

1000100

Swift/BAT
15 – 350 keV

5000.001

0.010.1

1101001000

T90 [s]Fig. 2. The distribution of T90 . The top panel shows distribution ofCGBM GRBs (mean is 8.9s). The middle high panel shows thatof BATSE GRBs (mean is 9.3s). The middle low panel shows thatof Fermi/GBM GRBs (mean is 13.4s). The bottom panel showsthat of Swift/BAT GRBs (mean is 24.4s).

was not in the normal operation. However, when GW
151226 occurred, CGBM was working. We checked TH
light curve data within a period of ± 60 s from the GW
trigger time. As the result, no significant signal was seenin the CGBM data. We also estimated 7σ upper limit
for GW 151226 assuming a spectrum of a typical short
GRB (α= −0.58, Epeak = 355 keV; Ghirlanda et al. 2009). The 7σ upper limit of HXM is 1.0 ×10−6 cm−2 s−1(7 - 500 keV) assuming 1 s integration time. The 7 σupper limit of SGM is 1.8 ×10−6 cm−2 s−1 (50 - 1000keV) assuming 1 s integration time (Adriani et al. 2016).
5. Summary
CGBM is performing a nominal observation since October 8, 2015. The current observation eﬃciency is ∼60%. CGBM detected 49 GRBs which consist of 40 long
GRBs and 9 short. The rate of short GRBs is ∼18 %.
The detection rate of GRB is 4 GRBs a month. 4 GRBs
were also detected by MAXI/GSC. The distribution of
the T90 obtained by CGBM is similar to that by BATSE.
CALET did not find a significant electro magnetic counterpart of GW151226 (Adriani et al. 2016 ).
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